An organic electroluminescence device, which includes an organic emitting layer between an anode and a cathode, has been known to emit light using exciton energy generated by a recombination of holes and electrons that have been injected into the organic emitting layer.
Such an organic electroluminescence device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
In order to use an emitting material in an organic electroluminescence device, a doping method for doping a dopant material in a host material has been known.
The doping method is designed so that excitons are efficiently generated by the injected holes and electrons and exciton energy is efficiently converted into light emission, where the exciton energy generated by the host is transferred to the dopant, so that light is emitted from the dopant.
Examples of a further improvement to be made in an organic electroluminescence device include improvements in emission lifetime and luminous efficiency, to which various studies have been made.
For instance, in order to enhance internal quantum efficiency, developments have been made on a phosphorescent material that emits light using triplet excitons. In recent years, there has been a report on a phosphorescent organic electroluminescence device (see, for instance, Patent Document 1).
With the use of such a phosphorescent material, 100% internal quantum efficiency can be theoretically achieved and an organic electroluminescence device with high efficiency and low power consumption can be obtained.
In order to intermolecularly transfer the energy from a phosphorescent host to a phosphorescent dopant (phosphorescent material) in a phosphorescent-emitting layer provided by doping the phosphorescent material, triplet energy gap (referred to as Eg(T) hereinafter) of the phosphorescent host material is required to be larger than the Eg(T) of the phosphorescent dopant.
Typically known material that exhibits effectively large Eg(T) is CBP.
By using CBP as the phosphorescent host, energy can be transferred to a phosphorescent dopant for emitting light of a predetermined emitting wavelength (e.g., green, red), by which an emitting device of high efficiency that shows phosphorescent emission can be obtained.
However, although an organic electroluminescence device in which CBP is used as the phosphorescent host exhibits much higher luminous efficiency due to phosphorescent emission, the organic electroluminescence device has such a short lifetime as to be practically inapplicable.
Accordingly, a material other than CBP that is usable as the phosphorescent host has been under development (see, for instance, non-Patent Document 1).
On the other hand, a variety of host materials for fluorescent dopants are known. Various proposals have been made on a host material capable of, with a combination of a fluorescent dopant, exhibiting excellent luminous efficiency and lifetime.
However, though a fluorescent host has a greater singlet energy gap Eg(S) than that of a fluorescent dopant, since the Eg(T) of the fluorescent host is not necessarily large, direct conversion of the fluorescent host to a phosphorescent host is not possible.
For instance, well-known examples of such a fluorescent host include an anthracene derivative, pylene derivative and naphthacene derivative. However, the Eg(T) of, for instance, an anthracene derivative is approximately 1.9 eV, which is insufficient for obtaining emission of visible light wavelength from 450 nm to 750 nm and is thus not suitable as a phosphorescent host.
On the other hand, it is known that an organic electroluminescence device with a long lifetime and a high efficiency can be obtained with the use of an emission layer containing a dopant in a host of a plurality of materials.
For instance, in Patent Document 2, a phosphorescent-emitting layer is provided using a phosphorescent host containing two or more hole-transferring substances to improve the efficiency and lifetime.    [Patent Document 1] US2002/182441    [Patent Document 2] JP-A-2006-135295    [non-Patent Document 1] Applied Physics letters Vol. 90.123509 (2007)